Conventionally, carbon fibers, glass fibers and the like are widely and generally used as a reinforcing material used for resins for molding materials. However, since carbon fibers are flame retardant, they are unsuitable for thermal recycling and are expensive. Glass fibers are relatively inexpensive, but have a problem in disposal.
Meanwhile, since plant fibers are relatively inexpensive and excellent in thermal recycling, development of techniques to utilize them as fillers for rubber materials has been investigated. However, hydrophilic plant fibers have poor dispersibility in hydrophobic rubber, so that plant fibers added to rubber aggregate and do not exert reinforcing effect, which cause deterioration of mechanical properties such as strength and the like conversely.
To such problems, various investigations have been made to improve dispersibility of plant fibers in rubber. In Patent Literature 1, fine plant fibers dispersed in water are mixed and stirred with a rubber latex, and water is removed to obtain a masterbatch having good dispersibility in rubber. In Patent Literature 2, affinity of fiber/rubber interface is enhanced by grafting monomers or polymers to plant fibers. However, in case of using these methods, several ten times amount of water relative to plant fibers is necessary for appropriately dispersing the micronized plant fibers, and after mixing with the latex, a large amount of water has to be removed, so that there is a problem of poor work efficiency.
Patent Literature 3 discloses a method of improving affinity of fiber/rubber interface by introducing vinyl groups to surface of plant fibers and further improving the affinity by bonding of the rubber and the fibers at the time of vulcanization. However, the plant fibers were not sufficiently dispersed.
In Patent Literature 4, a rubber composition is obtained by adding and mixing so that content of plant fibers is 5 parts by weight or more relative to 100 parts by weight of a diene rubber containing 20 parts by weight or more of a modified diene rubber having functional groups capable of bonding to hydroxyl groups on surface of dry plant fibers. However, the plant fibers did not reach a satisfactory dispersion state.